This invention relates to the removal of mercury (Hg) from gas streams, and in particular, to an apparatus and method for removing Hg, and in particular, elemental Hg from gas streams.
Mercury is a neurotoxin, which when emitted into the environment is of concern for both the health of man and other components of the biosphere. Even though the concentration of mercury in the atmosphere is low, it can be accumulated in the food chains of man and other species. The degree of environmental availability is dependent in part on the chemical form of mercury. Because of the widely recognized health impacts of this element, it has been identified as a key pollutant to be controlled. Some countries and states have banned the use of mercury in industrial processes, and the use of products containing mercury. As such, reduction of the amount of mercury from sources such as products like batteries and fluorescent light bulbs, and the emission of mercury from sources such as industrial processes, incineration, land fills and boilers is critical.
As mercury emission limits become more stringent, analytical methods capable of detecting ever lower mercury levels are required. In some cases, the required sensitivity exceeds that which can be achieved with known analytical methods. As a result, these measuring methods frequently require pre-concentration of the mercury-containing gas stream prior to analysis in order to measure low concentrations of mercury in typical gas streams.
To further complicate matters, mercury and some of its readily-formed compounds are relatively volatile. For example, mercury readily combines with chloride to form mercuric chloride (HgCl2). However, both elemental mercury and mercuric chloride have high vapor pressures at typical ambient and in-stack temperatures. As such, both elemental mercury and mercuric chloride vapor are difficult to remove from a gas stream for emissions control and/or subsequent analysis.
For all of these reasons, removal of mercury from gas streams has been extensively studied, and numerous methods have been reported that purport to effectively remove elemental mercury and mercury compounds from gas streams.
Mercury's propensity to combine with iodine has led to numerous efforts to remove mercury from gas streams by combining the Hg with iodine in some form. For example, Japanese patent JP 49-43197 teaches the removal of mercury from alkali electrolysis cell gas by contacting the gas with an iodide of Li, K, Ca, Mg, Zn, Al, Fe, Ni, or Co, or a compound of the general formula R4NI, where R is H or 1-4C alkyl containing no free I2, and which also includes at least one inorganic reducing agent. The carrier is carbon, a zeolite or silica gel.
JP 50-6438 teaches removal of Hg from gas streams by contacting the gas stream with a cationic exchange resin on which is adsorbed an iodide of Al, Zn, Sr, Fe, Na, Ni, Mg, Li, tetracthylamonium, methylene, naphthalene, o-phenol or benzene. Iodine is optionally loaded onto the resin in addition to the iodide compound.
U.S. Pat. No. 4,474,896 discloses an absorbent for mercury that is a polysulfide-containing adsorbent composition. In one embodiment, the adsorbent is a zeolite that has been treated to contain metal cation capable of forming insoluble polysulfides when exposed to sulfanes in either vapor form or in an organic solution.
While each of these methods is somewhat successful in removing Hg from gas streams, none is capable of doing so quantitatively; i.e., at low concentrations that might be present down stream at mercury controls and capture the mercury in a form that might be applicable to nondestructive analysis procedures such as x-ray fluorescence. As a result, none of these known methods are suitable for Hg removal to the levels required for either thoroughly removing Hg from a process or waste gas stream, for control of emissions, or for quantitative analysis of mercury-containing vapors in a gas stream.
It has been found that certain bio-membrane and resin media will efficiently removed oxidized forms of mercury from gas streams, but do not remove elemental mercury from these gas streams. The addition of chlorine gas to a gas stream containing elemental mercury will convert some of the elemental mercury to mercuric chloride, which could then be removed from the gas stream with the bio-membrane or resin media. However, the conversion of elemental mercury to mercuric chloride is inefficient and complete conversion can not be achieved under typical or generally applicable conditions.
It has also been found that elemental mercury can efficiently be removed from a gas stream by passing the gas stream through a bio-membrane or resin media that had been pre-treated with iodine. However, a limitation of this method is that the iodine on the filter can be transferred to the gas stream passing through the filter. Thus, the time of sampling through a fixed spot is limited to about an hour at normal operating conditions. Similarly, the volume and flow rates through the filter are limited not only by the pressure drop across the filter, but also the increased loss of iodine from the filter at higher volumes sampled. Another limitation of this method is that the gas stream may also contain high concentrations of condensable gases and the material must be used above the dew points for these gases. The high end of the temperature range for the filter is limited to about 200° F. to 250° F. Therefore, the effective temperature range within which the iodine-doped filter can be efficiently applied is limited to about 150° F. to 200° F.
It is desirable in the art to find alternative methods of trapping mercury on filters or other media for removing Hg from a process or waste gas stream, for control of emissions, or for quantitative analysis of mercury-containing vapors in a gas stream.